1. Field of the Invention
This invention relates to the use of optical-fiber pressure sensors to determine the direction of propagation of seismic pressure waves in a body of water.
2. Discussion of the Prior Art
In seismic exploration at sea, a plurality of pressure sensors are encased in a long tubular plastic streamer which may extend for one or two miles. A ship tows the streamer through the water at a desired depth. The earth layers beneath the sea are insonified by suitable means. The sonic waves are reflected from the earth layers below, to return to the surface of the water in the form of pressure waves. The pressure waves are detected by the pressure sensors and are converted to electrical signals. The electrical signals are transmitted to the towing ship via transmission lines that are contained within the streamer.
The reflected sound waves not only return directly to the pressure sensors where they are first detected, but those same reflected sound waves are reflected a second time from the water surface and back to the pressure sensors. The surface-reflected sound waves of course, are delayed by an amount of time proportional to twice the depth of the pressure sensors and appear as secondary or "ghost" signals. Because the direct and surface-reflected sound waves arrive close together in time--a few milliseconds -they tend to interfere with one another. It is desirable therefore to determine the direction of propagation of the sound waves so that the upward- and downward-propagating waves may be more readily sorted out during data processing.
It is possible to position two individual sensors in a fixed vertical array. It would of course then be easy to identify the direction of propagation of the sonic waves from the measured difference in time that a particular wavelet arrives at the respective sensors that make up the vertical array. See for example, U.S. Pat. No. 3,952,281. That method however requires two separate hydrophone cables. Since such cables cost about a half-million dollars each, that course of action would be decidedly uneconomical.
Assuming that sufficiently compact sensors could be obtained, it would be possible to mount a substantially vertical array of sensors inside the same streamer, a few inches apart. But a seismic streamer cable twists and turns as it is towed through the water. If a substantially vertical sensor array were to be mounted inside the streamer, there would be no way to determine which one of the sensors in the array is "up", assuming conventional detectors are used. It is also important to be able to identify unwanted waves travelling horizontally from scatterers within or near the bottom of the water layer.
As is well known, a water-pressure gradient exists between two points spaced vertically apart in a body of water. If then, there were some way that the hydrostatic pressure gradient between two vertically-disposed detectors could be measured, the uppermost detector of an array could be identified.
Conventional marine detectors or hydrophones use piezo-electric ceramic wafers as the active element. The wafers are generally mounted to operate in the bender mode. Transient pressure changes due to acoustic waves flex the wafers to generate an AC charge current. The wafers are also sensitive to hydrostatic pressure. But the DC charge due to hydrostatic pressure leaks off rapidly through associated circuitry. Therefore a differential DC component due to a hydrostatic pressure difference of the detector signal cannot be detected.
It is an object of this invention to provide a plurality of arrays of pressure sensors in an inexpensive streamer that is capable of detecting AC transient pressure signals due to seismic waves and to identify their direction of arrival with reference to the vertical whose direction is sensed by measuring the DC bias due to the vertical hydrostatic pressure gradient.